When sodium hydroxide is added to acetic acid this is the reaction:

CH₃COOH + NaOH → H₂O + NaCH₃COO

So the first addition of NaOH forms the salt, sodium acetate. But there is still some acetic acid since the titration is not complete. That makes a buffer! Titrations of weak acids (or weak bases) always show buffering behavior for this reason. And although we often say that you mix a weak electrolyte and a salt containing a common ion to make a buffer solution, it should be clear from this that buffers can also be made by partial neutralization of a weak acid or weak base.

The questions that remain about buffers are chiefly those that involve numbers. How can the pH of a buffer solution be predicted? What ingredients would be good for a buffer of pH 4? And so on. That's for next time.